Where do these bacteria come from?

Soon, however, that strain of bacteria showed up in dairy cows, which carried the bacteria harmlessly in their feces. From there, the bacteria entered the food chain, simply by what might have been slight contact. And slight contact, in the case of E. coli, is all that is needed to contaminate food.
     At that time, cattle were not inspected for E. coli, because it was thought to be harmless. But a single contaminated carcass in a slaughterhouse can be ground up with meat from hundreds of other cows to produce hamburgers.